This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. A cochlear implant (CI) is a prosthesis that helps deaf people perceive sounds;its main components are a microphone, a processor that converts sounds into electrical pulses, and an electrode array that stimulates the cochlear ganglion cells. The CI processor decomposes the speech signal into a number of spectral bands, extracts the bands temporal envelopes, which in turn modulate the amplitudes of biphasic electrical pulses applied to the ganglion cells. The electrode configuration can be monopolar (MP) or bipolar (BP). While in MP configurations the active and return electrodes are, respectively, inside and outside the cochlea, both electrodes are inside the cochlea in BP configurations. Because of the smaller spatial separation between active and return electrodes, the current spread is more localized in BP than in MP configurations, thereby enabling stimulation of specific ganglion cells along the basilar membrane. In theory, BP stimulation should achieve greater spatial (spectral) resolution, however, many CI users prefer MP to BP configurations. The effective current spread of MP and BP configurations has been estimated in psychophysical masking experiments (Chatterjee and Shannon 1998, 2004, Kwon 2006);the masking levels are the same for both configurations, suggesting that the effective current spread is also the same for both. However, physiological experiments using the same MP and BP current levels have shown that the spread of excitation in the auditory cortex is larger for MP configuration(Hughes 2006, Bierer and Middlebrooks 2002, Dingemanse and Frijns 2006). This discrepancy may arise because the psychophysical experiment measures loudness, and to achieve the same loudness a much smaller current level is needed with MP than BP configurations. In contrast, the physiological experiments measure the response of auditory cortex neurons to the electrical current applied to the ganglion cells;the current levels are the same for both configurations. In this research proposal we propose a new paradigm for the forward masking to determine the current spread in the MP and BP configurations. In order to measure the effective current spread of the MP masker and BP masker, the electrode configuration of the probe should be the same in both cases. This is different from the customarily forward masking experiments which use the same electrode configuration mode for probe and masker see Fig. 4. By setting the probe configuration to be fixed as BP then we eliminate the effect of the current spread at the probe. By fixing the probe configuration then we are comparing only the effect of masker current spread which could be MP or BP. To take into consideration the fact that MP requires less current to generate the same loudness as BP we will set the MP and BP masker current amplitude at levels that generate the same loudness. We hypothesis that the MP masking will be greater than for BP meaning that the current spread is larger for MP. If our hypothesis is correct, it will agree with the physiological measurements of the current spread inside the cochlea and the response pattern of the excited neurons in the auditory cortex.